Relatively small forged iron base metal articles can be produced from wrought stock or powdered metal, the latter being a relatively new technique of compacting metal powders.
To produce such an article of high Rc hardness, generally in the order of Rc 60, which means substantially high martensitic content, coupled with good surface finish and dimensional stability requires subjecting the wrought article after forging to at least the additional steps of cooling, machining, and rehardening. Each subsequent step is not only expensive but introduces different metal conditions which must be accounted for in the next following process step.
The forging of powdered metal preforms has consequently come into acceptance as a metal working process because of its chief advantage in eliminating the machining operations necessary with wrought material. In such process pre-alloyed metal powders are admixed with graphite, and lubricant compacted into briquettes, sintered, and forged. While desirable, the full benefits of this relatively new procedure have not been fully realized.
For example, in the manufacture of powdered iron base metal articles having hardness at the working surface of Rc 60 or thereabouts it has heretofore always been thought necessary to either (a) use a carbon content of 0.2-0.3 percent by weight and then carburize and subsequently harden the article, or b) use a carbon content of 0.5-0.7 percent and then austenize the article and quench it. While there are different specific ways to accomplish each processing method (a) and (b), these are well known in the art and it is sufficient for purposes of illustrating our invention to explain that while each achieves the surface hardness and surface finish desired, each such method causes undesired dimensional changes in the article itself due to the heat effect.